1. Field of the Invention
The present invention relates to a display device such as an organic electroluminescence display device or a liquid crystal display device, and to a wiring structure used therefore and a substrate for the display device.
2. Description of the Related Art
An organic electroluminescence (EL) display device has anodes formed in the pixel region and connected to the source electrodes of thin film transistors (TFTs) for driving pixels, organic EL layers (light-emitting layers) formed on the pixel electrodes and cathodes formed on the organic EL layers. A bottom emission structure which takes out light from the light-emitting layers onto the side of the substrate (anode side) was a main stream in the conventional organic EL display devices. The organic EL display device is driven by an electric current. In the organic EL display device of the active matrix type, therefore, each pixel requires 2 to 4 TFTs for driving the pixel. No light passes through the regions where the TFTs are formed. Therefore, the aperture ratio decreases in the pixels having an increased number of TFTs per a pixel. Relying upon the bottom emission structure, therefore, it is difficult to realize the organic EL display device having a high brightness.
In recent years, therefore, there has been developed an organic EL display device of the top emission structure which takes out light from the light-emitting layer onto the side opposite to the substrate (onto the cathode side)(JP-A-56-116072). FIG. 6 is a view illustrating in cross section the constitution of a pixel of an active matrix organic EL display device of a general top emission structure. Referring to FIG. 6, an underlying insulating layer 130 is formed on the whole surface on a glass substrate 110. A semiconductor layer 131 of a predetermined shape comprising polysilicon (p-Si) is formed on the underlying insulating layer 130. An insulating film 132 is formed on the whole surface of the substrate and on the semiconductor 131. A gate electrode 122 is formed on the insulating film 132. Of the semiconductor layer 131, a region just under the gate electrode 122 is serving as a channel region 131a. Of the semiconductor layer 131, the regions on both sides of the channel region 131a are serving as a drain region 131b and a source region 131c. An interlayer insulating film 134 is formed on the whole surface of the substrate and on the gate electrode 122. A drain electrode 124 and a source electrode 126 are formed on the interlayer insulating film 134. The drain electrode 124 is connected to the drain region 131b through a contact hole formed in the interlayer insulating film 134 and in the insulating film 132. Similarly, the source electrode 126 is connected to the source region 131c through a contact hole formed in the interlayer insulating film 134 and in the insulating film 132. An interlayer insulating film 136 is formed on the whole surface of the substrate on the drain electrode 124 and on the source electrode 126.
An anode 140 of molybdenum (Mo) is formed on the interlayer insulating film 136 in each pixel region. The anode 140 is connected to the source electrode 126 through a contact hole formed in the interlayer insulating film 136. An organic EL layer 142 is formed on the whole surface of the substrate and on the anode 140. A cathode 144 of aluminum (Al) having a very small thickness is formed on the whole surface of the organic EL layer 142 so as to permit the transmission of light. A transparent electrode 146 is formed on the whole surface of the cathode 144. Unlike the anode 140 formed for each pixel electrode, the cathode 144 and the transparent electrode 146 are formed on the whole surface so as to apply a common potential to all pixels. Therefore, the cathode 144 and the transparent electrode 146 are also called common electrodes.
Light emitted from the organic EL layer 142 include light (arrow a in FIG. 6) that directly passes through the cathode 144 and the transparent electrode 146 so as to be emitted onto the side opposite to the substrate, and light (arrow b in FIG. 6) reflected by the anode 140 and is emitted to the side opposite to the substrate. In the top emission structure, the anode 140 is formed even on a plurality of TFTs (only one of them is illustrated in FIG. 6) arranged in each pixel. Therefore, the aperture ratio is improved as compared to that of the bottom emission structure.
The anode of the organic EL display device is made of a material such as indium tin oxide (ITO), Mo or chromium (Cr) capable of efficiently injecting positive holes. On the other hand, the cathode is made of a material capable of efficiently injecting electrons. In the organic EL display device of the general bottom emission structure, there are arranged a transparent anode of ITO, an organic EL layer and a thick cathode film made of Al having a high reflection factor which does not permit light to pass through but reflects light, in this order from the side of the glass substrate 110. On the side of the substrate, there are taken out light that directly passes through the transparent anode and light that is reflected by the cathode of Al and, then, passes through the anode.
In the top emission structure, on the other hand, light must be taken out through the cathode 144 of Al. It is, further, necessary to use, as a reflection electrode, the anode 140 made of ITO, Mo or Cr without having so high reflection factor. Therefore, though the aperture ratio of the pixels is improved, the organic EL display device of the top emission structure is accompanied by a problem in that it is difficult to enhance the overall efficiency for taking out light.
Here, in the TFT substrate using amorphous silicon (a-Si) as the semiconductor layer for operating TFTs, the driver IC is mounted by the TAB (tape automated bonding) system. On the contrary, in the TFT substrate using p-Si having a high mobility as the semiconductor layer for operation, it is allowed to form not only the TFTs for driving the pixels but also the TFTs for peripheral circuits such as gate drivers and data drivers in the frame regions surrounding the display region. In recent years, there have been increasingly used display devices of the type of integrated peripheral circuits integrally forming the TFTs for driving pixels and the TFTs for peripheral circuits on the same substrate.
The display device of the type of the integrated peripheral circuits requires a multi-layer wiring technology for forming complex peripheral circuits yet realizing narrow frame. In the transmission type liquid crystal display device, for example, an ITO is used as a material for forming the pixel electrodes. The ITO has a resistance higher than those of other metals, and is not suited for use as a wiring for the peripheral circuits. As the wiring, therefore, there are chiefly used two kinds of materials (two layers), i.e., a material forming the gate bus lines and gate electrodes, and a material forming drain bus lines and intermediate electrodes. However, complex circuit constitution may often require wirings of three or more layers.
Of the above two kinds of materials, further, the material forming the gate bus lines must withstand a heat treatment of a high temperature (e.g., activation of impurities injected into the semiconductor layer) at a subsequent step. In many cases, therefore, there is used a refractory metal or p-Si to which impurities are added. The refractory metal or p-Si has a relatively high resistance. Therefore, the material having a relatively low resistance is limited to a material for forming drain bus lines. Therefore, there remains a problem in that the wirings of low resistances cannot be solidly crossed via an insulating film in the peripheral circuits.
The display devices which are not of the type of integrating the peripheral circuits do not require the multi-layer wiring technology, and the above problem does not occur. Further, the display device which uses a-Si as the semiconductor layer for operating the TFTs does not have a step of heat-treatment at a high temperature, permits a low-resistance metal such as Al having a low melting point to be used as a material for forming gate bus lines and, hence, makes it possible to solidly cross the wirings having low resistances. As the panel sizes are ever increasing, however, it is urged to lower the resistance of the gate bus lines even in these display devices.
Incidentally, the document of the related art is as follows:    [Patent Document] JA-A-56-116072